Micro-optical diffraction grid and process for producing the same

ABSTRACT

The invention relates to microoptical diffraction gratings for electromagnetic radiation and to a method suitable for the manufacture thereof. The diffraction gratings in accordance with the invention can in particular be utilized for use as microspectrometers which can be used in this connection in the form of scanning microgratings. In accordance with the object set, they should be provided with improved surface topology and should be able to be manufactured cost effectively and in high volumes. With the diffraction gratings in accordance with the invention, a surface structure is formed at a surface of a substrate and is formed from linear structural elements arranged equidistantly and aligned parallel to one another. In addition, the total surface of the substrate and of the structural elements is coated with at least one further layer which forms a uniform sinusoidal surface contoured in wave shape and having alternatingly arranged wave peaks and wave troughs. With reflection gratings, a reflective layer can additionally be applied to increase the intensity of reflected radiation.

The invention relates to microoptical diffraction gratings forelectromagnetic radiation and to a method suitable for the manufacturethereof. The diffraction gratings in accordance with the invention canin particular be utilized for use as microspectrometers which can beused in this connection in the form of scanning microgratings.

Such microspectrometers with pivotable diffraction gratings have beendescribed, for example, by H. Grüger et al. in “Performance andApplications of a spectrometer with micromachined scanning grating”;Micromachining and Microfabrication, part of SPIE Photonic West (2003).

Very small micromechanical systems are desired for a number ofapplications; accordingly, the diffraction gratings used therein must beprovided in correspondingly small form. As already indicated, in thisprocess, the diffraction gratings are pivoted around an axis of rotationand the electromagnetic radiation which is directed onto such adiffraction grating from a corresponding radiation source is guidedsequentially in a spectral range via one or more detectors suitable forthe detection of specific wavelengths of the electromagnetic radiation.

Usually, highly precise and efficient diffraction gratings aremanufactured by a casting process from a so-called master or also byholographic processes. For the forming from a master, the latter must bemanufactured in advance. In this connection, the manufacture takes placesuch that equidistant lines are formed in a substrate, which consistse.g. of metal, by means of a scoring tool. The forming from such amaster can then e.g. take place by means of a hardening plastic, e.g.from epoxy resin. Subsequent to the forming, a metallic layer of highreflectance can be applied to such a formed structure.

It is, however, problematic in this connection that a considerablemechanical pressure is required for the forming and that considerablecompressive forces act on the substrates which typically have athickness of only some few 10 μm. Problems moreover occur with therequired lateral adjustment precision.

In addition, the possible number of pieces of individual diffractiongrating elements of such a forming from a master is limited. Theproduction costs of such diffraction gratings suitable formicromechanical applications are naturally thereby increased.

Holographic processes for the manufacture of corresponding diffractiongratings are based on the interference principle with a use of laserradiation. An intensity profile arises by interference of partial laserrays which is sinusoidal to some extent and with which thephotosensitive layer on a substrate is illuminated with thecorresponding interference pattern. This interference intensity profileis then transferred onto the photosensitive layer in topological formafter the exposure and subsequent development. The photosensitive layercan subsequently be coated with a highly reflective metal film.

However, the installation technology such as is usually used in a matureform in semiconductor manufacture cannot be used in the manufacture sothat an additional implementation in such an installation technology isrequired.

It is moreover known to manufacture diffraction gratings with acorresponding surface topology by the process techniques known per se ofgray-scale lithography. In this connection, however, the number/spacingof the individual lines of a diffraction grating is limited so that thespectral resolution of such a diffraction grating is likewise limited.

Diffraction gratings can, however, also be provided by a simplestructuring of a reflective layer applied to a substrate. In thisconnection, a rectangular diffraction grating can be obtained in a firstapproximation. The diffraction gratings manufactured in this way,however, have a low effectiveness and can accordingly only be used forspectral analysis with high-intensity sources for electromagneticradiation.

It is therefore the object of the invention to provide efficientmicrooptical diffraction gratings with a suitable surface structurewhich can be manufactured in a cost-effective manner and in highvolumes.

In accordance with the invention, this object is solved by amicrooptical diffraction grating having the features of claim 1. Theycan be manufactured using a method in accordance with claim 13.

Advantageous embodiments and further developments of the invention canbe achieved using the features designated in the subordinate claims.

The diffraction gratings for electromagnetic radiation in accordancewith the invention are made such that a surface structure is formed at asurface of a substrate.

This surface structure consists of linear structural elements which arearranged equidistant from one another and which should moreover bealigned parallel to one another. The linear structural elementsaccordingly form elevated portions at the respective surface of thesubstrate. This can be achieved by forming likewise linear recesses inthe surface.

At least one layer is then formed over the whole surface of thesubstrate, that is, also over the surfaces of the structural elements,said layer forming a uniform sinusoidal surface contoured in wave shapeand having alternatingly arranged wave peaks and wave troughs. Such awave-shaped surface contour can be formed independently of the linearsurface structure in the formation of the at least one layer since arounding effect can be utilized in the coating technologies which can beused for the manufacture of diffraction gratings in accordance with theinvention.

The cross-sectional geometry in which the structural elements are formedon the respective surface of a substrate is thus insignificant to alimited degree. Structural elements can thus have triangular,rectangular or also trapezoidal cross-sectional shapes havingcorresponding edge regions and nevertheless an almost continuouswave-shaped surface structure can be formed.

At least partly elliptical cross-sectional shapes of structuralelements, which can be formed, for example, by lateral etching, whichwill be looked at later in the following, can also be easily controlledin the formation of the wave-shaped surface contour.

Advantageously, the at least one or more individual layers formed overone another form a sinusoidal surface. This can in particular beachieved in that at least one layer is made from a material or from amaterial mix which is plastically deformable by an energy input. Theenergy input should preferably be carried out after the formation of thelayer(s). In this connection, the viscosity can be reduced to the extentthe material/material mix flows and deforms in so doing. The deformationis maintained after the end of the energy input. A much more regularizedsurface topology can thereby be achieved which is made at least almostsinusoidal and very uniform wave peaks and wave troughs with convex orconcave curvatures can be formed.

Suitable materials or material mixes are, for example,borophosphosilicate glass (BPSG), metals, e.g. Al, Ni, Au, Ag, Cr, Cu oralso metal alloys such as AlSiCu, AlCu or polymers such as BCB, PMMA,SU-8 or photoresists (e.g. AZ7212, AZ 7217).

The input of energy can take place in different forms. A radiation withelectromagnetic waves which are preferably absorbed by the respectivematerial or material mix can thus be used.

A thermal treatment can, however, also be carried out in a differentfashion by annealing in a furnace.

There is, however, also the possibility of introducing the energy inputby means of electrical resistance heating or induction, whereby thenelectrically conductive parts can be connected to an electrical voltagesource in suitable form or a coated substrate can be exposed to anelectrical or electromagnetic alternating field.

The plastic deformability can, however, also be achieved by chemicalactivation of a material or material mix as a result of the introducedenergy.

The surface of the substrate on which the structural elements arearranged can in particular be smooth and planar on the use ofdiffraction gratings in accordance with the invention for use in apredetermined spectral range of electromagnetic radiation.

There is moreover the possibility to provide the diffraction gratings inaccordance with the invention as transmission gratings or also asreflection gratings.

With a transmission grating, at least one layer, e.g. made from therespective substrate material, should then be applied to a substratetransparent for the respective radiation range and the wave-shapedsurface contour should be formed with this at least one layer.

In the case of reflection gratings, such a layer can be formed from amaterial which reflects the respective electromagnetic radiation, withthere also being the possibility of forming a plurality of suchreflective layers over one another. Highly reflective metals or metalalloys can thus be used for such layers, for example. Aluminum, silver,gold or a corresponding alloy should be named by way of example here.

In the event that a plurality of layers should be formed over the totalsurface of a diffraction grating in accordance with the invention, saidlayers do not necessarily have to be formed from correspondinglyreflective materials. There is thus the possibility of formingcorresponding reflective multilayer systems of alternatingly arrangedlayers of a respective material with a higher optical refractive indexand of a material with a lower optical refractive index. Such amultilayer system is then likewise able to form a reflection grating.

In this connection, however, interference can also be used and therespective layer thicknesses of such layers of multilayer systems forpresettable wavelengths can each be formed as so-called λ/4 layers, withthe respective layer thicknesses then taking up a whole number multipleof λ/4 of a correspondingly predetermined wavelength. In thisconnection, the respective angle of incidence of the correspondingelectromagnetic radiation onto the radiated surface of the diffractiongrating is naturally a parameter to be taken into account.

With the microoptical diffraction gratings in accordance with theinvention, a matching to selected wavelength spectra such as extremeultraviolet (EUV), deep ultraviolet (DUV), ultraviolet, visible light,near infrared (NIR) and infrared is possible.

The diffraction gratings in accordance with the invention can bemanufactured such that a layer, for example a photoresist layer, isformed on a surface of a substrate and the photoresist is structured bya photolithographic process with subsequent developing so that in afollowing etching step, e.g. by known dry physical methods or drychemical methods or wet chemical methods, linear recesses can be formedin the substrate and thereby the structural elements at the substrate.In this connection, use can be made of conventional installationtechnology such as is usually used in the semiconductor industry.

A structuring can thus be obtained with current technology with linearstructural elements of more than 5000 on 1 mm.

A specific preselected surface topology with a suitable cross-sectionalprofile can be formed in a reproducible manner.

A substrate pretreated in this manner can then, as already addressed ingeneral form, be coated with at least one layer which then forms thewave-shaped surface contour. PVD or CVD processes known per se can beused for the forming of the layer.

There is thus easily the possibility of simultaneously processing acorrespondingly large-format diffraction grating or a plurality ofsmall-format diffraction gratings on a substrate in one respectivetechnological step, whereby the single piece costs can be considerablyreduced over conventional solutions.

Furthermore, atoms of foreign elements can be implanted into at leastone layer. This results in adapted or optimized flow properties,strains, stress or adapted thermal coefficients of expansion.

It can moreover be advantageous additionally to form at least one layeron a side of the substrate. The residual stress relationships canthereby be influenced. There is the possibility of thereby compensatingresidual stresses present in advance.

A direct deformation of the diffraction grating can, however, also beachieved by one or more layer(s) formed at the substrate at least oneside. For example, an arching of the structured surface can thus becompensated and a smooth planar surface can be achieved, with theexception of the surface topology.

However, a concave or convex arching/curvature of the structured surfacecan also be achieved by layers having layers formed at a side and actingon the substrate to influence the optical properties, e.g. the focallength.

In this connection, the stress relationships and, where necessary, thearching/curvature can be selected for a diffraction grating inaccordance with the invention formed in this manner while taking accountof the respective operating temperature range.

This can be influenced, for example, by a suitable selection of thelayer materials with corresponding thermal coefficients of expansion, ofthe number and/or of the thickness of layers for at least one side ofsubstrates.

The invention will be explained in more detail by way of example in thefollowing.

There are shown:

FIG. 1 a partial section of an example for a diffraction grating inaccordance with the invention, as a reflection grating, in a schematicrepresentation; and

FIG. 2 a partial section of a further example in a schematicrepresentation.

In the form shown in FIGS. 1 and 2, there is the possibility of formingrecesses photolithographically in a substrate 1 made of silicon, saidrecesses being linear after an etching step and forming structuralelements 2 at the surface of the substrate 1. The linear structuralelements 2, which are aligned parallel with one another, have atrapezoidal (FIG. 1) or rectangular (FIG. 2) cross-section. Thestructural elements 2 have a height h1 and a structural element width d.The structures described repeat periodically.

Subsequently, a highly reflective layer 3 of aluminum can be formed, bymagnetron sputtering for example, over the total surface of thesubstrate 1, that is also above the structure elements 2. The depositedlayer 3 forms a surface contour in wave shape so that between thestructural elements 2 in troughs it had a layer thickness h2 in themiddle between two adjacent structural elements 2 and above structuralelements 2 a height H. A sinusoidal surface structure was able to beachieved after formation of the layer 3.

In the example shown in FIG. 1, linear structural elements 2 having atriangular cross-section were formed by wet chemical etching oranisotropic etching on the surface of a substrate 1 which was formedfrom (100)-silicon. On a variation of the parameters or of the substrateorientation, however, other cross-sectional elements for structuralelements 2, for example rectangular cross-sections, as in the example ofFIG. 2, can also be formed.

A layer 3 of borophosphosilicate glass (BSG) was deposited on asubstrate 1 prepared in this way and the surface contour formed usingthe structural elements 2 was mapped or rounded with larger layerthicknesses. Subsequently, the coated substrate 1 was annealed and afurther plastic deformation of the layer 3 was achieved by the heating,which resulted in a sinusoidal surface contour on the surface of thelayer 3 with alternatingly arranged wave peaks and wave troughs whichare arranged between the structural elements 2.

At least one further layer 4, for example of silicon nitride, can beapplied to the layer 3 to achieve a further compensation of residualstrains.

A reflective layer 5 can be applied directly to the layer 3 or, as shownin FIGS. 1 and 2, also applied to a layer 4. The layer 5 has beendeposited from aluminum here.

The thicknesses d3, d4 and d5 of the layers 3, 4 and 5, the geometry,the dimensioning a, b and h1 as well as the spacings of the structuralelements 2 have been selected in this context so that a sinusoidalsurface topology and freedom from residual strain were able to bereached at the surface of the diffraction grating.

1. A microscopic diffraction grating for electromagnetic radiation, wherein a surface structure is formed at a surface of a substrate and is formed from linear structural elements arranged equidistantly and aligned parallel to one another; and the total surface of the substrate and of the structural elements is coated with at least one further layer, with the at least one further layer forming a uniform surface contoured sinusoidally in wave shape and having alternately arranged wave peaks and wave troughs.
 2. A diffraction grating in accordance with claim 1 wherein at least one of the layers is made from at least one of a material and a material mix which is plastically deformable under the effect of energy.
 3. A diffraction grating in accordance with claim 1 wherein the surface of the substrate at which the structural elements are formed is made as a smooth planar surface.
 4. A diffraction grating in accordance with claim 1 wherein the at least one further layer reflects electromagnetic radiation.
 5. A diffraction grating in accordance with claim 4 wherein the at least one layer is formed from at least one of a highly reflecting metal and a metal alloy.
 6. A diffraction grating in accordance with claim 5 wherein the layer is made from at least one of aluminum, silver, gold and alloys thereof.
 7. A diffraction grating in accordance with claim 1 wherein a plurality of layers form a multilayer system formed form alternately arranged layers of materials having higher and lower optical refractive indices.
 8. A diffraction grating in accordance with claim 1 wherein the at least one layer has a layer thickness corresponding to a whole number multiple of λ/4, λ being a predetermined wavelength.
 9. A diffraction grating in accordance with claim 1 wherein the structural elements have one of a triangular cross-section, a rectangular cross-section, a trapezoidal cross-section and a cross-section made at least partly in the form of an ellipse.
 10. A diffraction grating in accordance with claim 1 wherein atoms of other elements are implanted in at least one of the layers.
 11. A diffraction grating in accordance with claim 1 wherein at least one layer is arranged on the rear side of the substrate.
 12. A diffraction grating in accordance with claim 11 wherein the substrate with the at least one layer formed on it is curved.
 13. A method for the manufacture of a diffraction grating in accordance with claim 1 wherein the surface of a substrate with linear structural elements arranged equidistantly thereon is coated at least regionally with at least one further layer and a uniform surface contoured sinusoidally in wave shape is obtained.
 14. A method in accordance with claim 13 wherein linear recesses are formed in a surface of the substrate and the structural elements are thus formed at the surface.
 15. A method in accordance with claim 13 wherein the arching of the surface is regularized and adapted to the sinusoidal shape by applying energy to the at least one further layer and by the resulting plastic deformation of the at least one of a material and a material mix from which the at least one layer is formed.
 16. A method in accordance with claim 15 wherein a subsequent heating is carried out.
 17. A method in accordance with claim 15 wherein the heating is carried out by at least one of radiation, electrical resistance heating and inductively.
 18. A method in accordance with claim 13 wherein the structural elements are formed on the surface of the substrate by at least one of a dry chemical etching process, a dry physical etching process and a wet chemical etching process.
 19. A method in accordance with claim 1 wherein the residual stress is influenced in a defined form with at least one layer applied to at least one of the front side and the rear side of the substrate.
 20. A method in accordance with claim 19 wherein the residual stresses are compensated in the operating temperature range of the diffraction grating.
 21. A method in accordance with claim 20 wherein the residual stress in the operating temperature range of the diffraction grating is set such that said diffraction grating is curved and the structured surface is one of planar, arched concavely and arched convexly.
 22. A method in accordance with claim 1 wherein at least one of the rear side of the substrate and at least one of the layers applied to the rear side is structured for the defined influencing of residual stress. 